The present invention relates to a liquid crystalline material having a lamellar layered or stratified structure, as well as to a device for modulating light which has a liquid crystalline material having a lamellar layered or stratified structure.
It is believed that current trends of transmitting and processing information optically have significantly increased the need for components for controlling the properties of light beams. In this context, varying the phase of light is also of fundamental importance for many adaptive optical systems, for example, astronomical or terrestrial imaging systems, optical communications systems, or for systems used to simulate turbulence. Available dielectric, electrooptic crystals, for example, LiNbO3, which can be used, for example, for wavefront correction, may exhibit long switching times, small apertures, and minimal variations in refractive indices, the latter necessitating high switching voltages to set a predefined phase modulation.
Recently, interest has intensified in liquid crystals as promising materials for use in wavefront control. In this context, the optical properties are electrically controlled, for example by orienting and deforming the local index ellipsoid of the material. It is believed that some important electrooptic effects in liquid crystals manifest themselves in a simultaneous change in the phase and polarization of the light transmitted through the liquid crystal, the change in polarization, however, being an undesirable secondary effect, for example, in adaptive optical systems and in most other phase modulation applications.
It is believed that the electrically induced transition from a homogeneous, planar orientation to a homeotropic orientation in nematic liquid crystals, described as the S-effect, may bring about a pure phase modulation when linearly polarized light is used, whose polarization plane is situated in parallel to the director of the liquid crystal in the homogeneous, planar orientation. Materials of this kind may be employed in pixelated, electrically controlled, spatially resolved phase modulators, but also in optically addressable variants, which additionally have a photoconductive layer. In both cases, several ten milliseconds may be needed to achieve a phase modulation of 2xcfx80, higher phase delays requiring relatively large layer thicknesses of about 5 xcexcm and an electrically controllable refractive index variation of about xcex94n=0.15. Since switching times increase with cell thickness, the most often used wavefront correctors, which work on the basis of nematic liquid crystals, have a pulse frequency within the range of a few Hz.
In comparison, chiral-smectic, ferroelectric liquid crystals render possible switching times in the microsecond range. Available methods heretofore essentially consider two effects in the use of ferroelectric liquid crystals of this kind. U.S. Pat. Nos. 4,838,663 and 4,563,059 discuss a bistable switching between two surface-stabilized orientation states, and Swiss Patent No. CH-3722/87 discusses the deformation of the helical superstructure. While in the first case, virtually only the optical axis is rotated and no change in the refractive index occurs, in the second case, a rotation of the optical axis and a variation in the refractive index occur simultaneously. Due to the rotation of axes, the effects in the available chiral-smectic, ferroelectric liquid crystals always result in a polarization change in the light transmitted through the liquid crystal, which makes the available ferroelectric crystals appear to be unsuited for pure wavefront corrections.
Polyphilic liquid crystals, whose molecules may be composed of a rigid, readily polarizable central group including two or three phenyl rings, and of two, more flexible wing groups, in particular aliphatic chains, are also available. When an aliphatic wing group is replaced by a perfluorinated chain, a polyphilic separation of the molecular components may be achieved, the molecules thereby ordering themselves in the liquid crystalline state such that the perfluorinated groups predominately face in the same direction. It is believed that this behavior was verified in the reference, xe2x80x9cFerroelectricsxe2x80x9d, Tournilhac et al., volume 114, pages 283-287, 1991 and in xe2x80x9cLiquid Crystalsxe2x80x9d, volume 14, pages 405-414, 1993. In the investigated material F(CF2)8(CH2)11xe2x80x94Oxe2x80x94Phxe2x80x94Phxe2x80x94CN, one ascertained an optical tilt angle xcex8 of 48xc2x0 and an inclination angle xcex8F of the fluorinated chain of 28xc2x0; in the material F(CF2)8(CH2)11xe2x80x94Oxe2x80x94Phxe2x80x94NPhxe2x80x94CN, an optical tilt angle xcex8 of 49xc2x0 and an inclination angle xcex8F of the fluorinated chain of 30xc2x0; and in the substance F(CF2)8(CH2)11xe2x80x94Oxe2x80x94Phxe2x80x94Phxe2x80x94COxe2x80x94Oxe2x80x94CH2CF3, an optical tilt angle xcex8 of 51xc2x0 and an inclination angle xcex8F of the fluorinated chain of 33xc2x0; Ph being an abbreviation for a phenyl ring and NPh for a pyridine ring. For the mentioned polyphilic liquid crystals having perfluorinated chains, the references discusses that the optical tilt angle xcex8 specific to the rigid central group and the inclination angle xcex8F of the perfluorinated chain differed from the layer normal z. Further, when perfluorinated chains are used, there may be a reduction in the molecular rotational viscosity of ferroelectric liquid crystals, so that low switching times within the range of less than 15 sec may be attainable.
However, the discussed liquid crystals may have some generally occurring polarization modulation, which may not be acceptable for most phase modulation applications.
An exemplary embodiment of the present invention is directed to providing a liquid crystalline material having a lamellar structure which can be used exclusively for phase modulation.
Another exemplary embodiment of the present invention is directed to a new liquid-crystal material class having a lamellar layered structure, the liquid crystal has arcuated or angular dimer molecules, which each include two central units, the longitudinal axes of the two central units having at least approximately opposite inclination angles, for example, opposite with respect to the layer normal. With the arrangement or system in accordance with the present invention of the two central units of the dimer molecule, the molecular index ellipsoid, which is substantially composed of the components of the two readily polarizable central units, may be positioned such that the optical axis is always parallel to the layer normal, so that a polarization modulation of light may be prevented, which is transmitted in a defined direction, perpendicularly to the layer normal through the liquid crystalline material.
To maximize the desired phase modulation, depending on the specific embodiment of the present invention, the amount of the two inclination angles may be within the range of about 10 to 90xc2x0. In order to construct the dimer molecule and thereby form a vertex or peak, the two central units may be bonded or joined together, it being possible for the bond to be formed by at least one neutral, molecular chain. Depending on the specific embodiment of the present invention, the vertex between the two central units of the dimer molecule can be an angle apex or, in the case that an arc may be formed between the central units, it may also be an arc midpoint.
The formation (or configuration or geometry) of the liquid crystalline material in accordance with the present invention can be applied to a multiplicity of thermotropic and lyotropic liquid crystals. In this context, the vertices (or peaks) of the dimer molecules of adjacent layers can be nearly unidirectional or, in another specific embodiment, approximately directed in opposite directions, thereby forming a double-layer structure.
To produce a helical structure having a predefined pitch, which can be canceled by applying an external electric field, so that a phase modulation is adjustable, the azimuth angles of the vertices of successive layers may change uniformly by a predefined value, so that the vertices rotate along the z-direction. However, to form a helical structure having double layers and a predefined pitch, the azimuth angle of a pair of vertices of successive double layers may be affected by a predefined value, so that the pairs of vertices rotate along the z-direction.
To enable an external electric field to act upon the arcuated or angular dimer molecules, the molecules each exhibit a transverse dipole moment. If the dipole moment is orthogonal to the optical axis, i.e., to the layer normal, the electric field may function optimally to modify the helical structure and, thus, to produce a pure phase modulation.
To produce the described helical structure, the dimer molecules may have at least one chiral center, in particular outside of the rigid central units. Providing a chiral center of this kind in the dimer molecule may allow that in an interplay with other molecular components of the dimer, it is able to produce a permanent dipole moment. To form specific helical structures, each of the dimer molecules may have two chiral centers.
To construct the dimer molecules of the liquid crystalline material in accordance with the present invention, the molecules between the two central units, i.e., as wing groups, can have at least one aliphatic fragment. In accordance with the present invention, the aliphatic fragments can be made of carbon chains in the form xe2x80x94(CH2)nxe2x80x94, n being within the interval from 0 to 16.
To ensure that a spontaneous polarization of the liquid crystalline material of the present invention may be produced macroscopically, the dimer molecules may each contain at least one polyphilic fragment that is located asymmetrically with respect to the vertex of the molecule. An effect of the asymmetrical, polyphilic molecular structure may be that a parallel positioning (or configuration) of adjacent molecules is preferred within one layer over an antiparallel positioning, so that there is spontaneous polarization within one layer. Thus, it may prevent the dipole moments of two adjacent, oppositely oriented molecules from being mutually compensated since both possible orientations are otherwise energetically equivalent and, therefore, on the average, are occupied with the same rate of occurrence. Since the oppositely oriented molecules exhibit dipole moments having opposite signs, no spontaneous polarization occurs in the case of the antiparallel positioning. In accordance with the present invention, the asymmetrical, polyphilic molecular structure can be achieved in that the polyphilic fragment has at least one perfluorinated chain of the form xe2x80x94(CF2)nxe2x80x94, where n can lie between 4 and 16.
Depending on the specific embodiment of the present invention, in the absence of an external electric field, the pitch of the helical structure can be within the range of between 50 to 1400 nm.
The liquid crystalline material in accordance with the present invention may be used for the phase modulation of light, for example, in an appropriate device. A device of this kind may be distinguished by the ability to generate an electric field substantially perpendicularly to the layer normal, and by the fact that the light is transmitted in a direction perpendicular to the layer normal through the liquid crystalline material. In this manner, besides the desired phase modulation, the light may be prevented from undergoing a polarization modulation. Using the liquid crystalline material of the present invention, one can construct phase modulators of the reflective type, as well as of the transmission type. In addition, the phase of light can be modulated as a spatially resolved phase using a multi-pixel structure. Also, the physical properties of the liquid crystalline material may render possible optically addressable, spatially resolved phase modulators, in which a photoconductive layer may be used to produce a spatially resolved external field with the assistance of incident light radiation.